Thick film dielectric electroluminescent devices as described in U.S. Pat. No. 5,432,015 (the entirety of which is incorporated herein by reference) include a thick film dielectric structure that provides for superior resistance to dielectric breakdown as well as a reduced operating voltage as compared to TFEL displays. The thick film dielectric structure when deposited on a ceramic substrate withstands somewhat higher processing temperatures than TFEL devices, which are typically fabricated on glass substrates. This increased high temperature tolerance facilitates annealing of phosphor films at higher temperatures to improve their luminosity. However, even with this enhancement, improvements in display luminance and colour coordinates are desirable in order to keep pace with ongoing improvements to cathode ray tube (CRT) displays, particularly with recent trends in CRT specifications to higher luminance and higher colour temperature.
A high luminosity full colour thick film dielectric electroluminescent display requires that the thin film phosphor materials used for the red, green and blue sub-pixels be patterned so that the emission spectrum for each colour of pixel is tailored to minimize the attenuation associated with the optical filters needed to achieve the required colour coordinates for each sub-pixel. For relatively low-resolution displays patterning can be achieved by depositing the phosphor materials through a shadow mask. However, for high resolution displays the shadow mask technique does not provide adequate accuracy requiring that photolithographic methods be employed. Photolithographic techniques, as exemplified in U.S. patent application Ser. No. 09/540,288 (the entirety of which is incorporated herein by reference) require the deposition of photoresist films and the etching or lift-off of portions of the phosphor film to provide the required pattern. Such a patterning process typically involves the initial deposition of one phosphor material for one of the red, green or blue sub-pixels, followed by deposition of a photoresist layer that is patterned so that selected portion of the deposited phosphor film can be etched away above the other two sub-pixel locations where the first-deposited phosphor film is not desired. The second two phosphor layers can be patterned using a lift-off process, due to the underlying surface relief of the initial patterned layer that allows for the exposure of the edges of the photoresist film that underlies the second and third phosphor layers on their respective pixels. This process requires the use of a solvent that dissolves the photoresist film, thus allowing those portions of the phosphor layer overlying the resist to be lifted off without being dissolved. However, the initial phosphor film generally needs to be removed by a direct etching process where the overlying resist has been removed. This requires that the phosphor material be soluble in the etchant at that stage in the process. Stable phosphors are not typically very soluble, and it is desirable to have a phosphor in a precursor form deposited as a film that is not fully reacted that can be further processed to form the final stable phosphor material following the etching step. Nevertheless, the phosphor film in its precursor form must not react in an unfavourable way with the process environment or the etchant during the etching process. The phosphors and methods for processing these phosphors as described in the prior art to not fully meet these requirements and therefore the performance or stability of the final phosphor may be adversely affected.
Cerium activated strontium sulfide for blue phosphors and manganese activated zinc sulfide for red and green phosphors are typically used in full colour electroluminescent displays. The optical emission from these phosphor materials must be passed through an appropriate chromatic filter to achieve the necessary colour coordinates for red, green and blue sub-pixels, resulting in a loss of luminance and energy efficiency. The manganese activated zinc sulfide phosphor has a relatively high electrical to optical energy conversion efficiency of up to about 10 lumens per watt of input power and the cerium activated strontium sulfide phosphor as an energy conversion efficiency of 1 lumen per watt, relatively high for blue emission. However, the spectral emission for these phosphors is quite wide, with that for the zinc sulfide based phosphor material spanning the colour spectrum from green to red and that for the strontium sulfide based material spanning the range from blue to green, thus necessitating the use of the optical filters. The spectral emission of the cerium activated strontium sulfide phosphor can be shifted to some degree towards the blue by controlling the deposition conditions and activator concentration, but not to the extent required to eliminate the need for an optical filter.
Alternate blue phosphor materials have been evaluated that have narrower emission spectra tuned to provide the colour coordinates required for blue sub-pixels. These include cerium activated alkaline earth thiogallate compounds. These give good blue colour coordinates, but have relatively poor luminosity and stability. Since the host materials are ternary compounds, it is relatively difficult to control the stoichiometry of the phosphor films. Europium activated barium thioaluminate phosphors provide excellent blue colour coordinates and higher luminance, but again as a ternary compound, its stoichiometry is difficult to control.
Various methods have been developed in order to deposit phosphor films yielding a high luminosity. One such method is the vacuum deposition of phosphor films comprising europium activated barium thioaluminate. This is accomplished from a single source pellet using sputtering or electron beam evaporation however, this method has not yielded films with high luminosity. Improved luminance of barium thioaluminate phosphors has been achieved by using a hopping electron beam deposition method to deposit films from two source pellets. The stoichiometry of the deposited phosphor film is controlled by controlling the relative dwell time of the electron beam impinging on each of the two source materials. This method is not however, readily scalable to facilitate commercial production of large area displays and furthermore it cannot be controlled to compensate for changes in the evaporation rates from the two sources as the deposition proceeds and the source pellets are depleted.
Another method that has been adopted to improve the stoichiometry of deposited thioaluminate phosphors is to use more than one source for the deposition requiring added controls over the relative deposition rates for the different sources. The required relative evaporation rates must be calibrated for each specific piece of deposition equipment and the requirement for multiple sources constrains the design of the deposition equipment, generally adding to the cost of the equipment. Further, evaporation methods are not well suited for the deposition of large area films such as a required for the fabrication of large electronic displays such as those for wall television applications.
U.S. patent application Ser. No. 10/036,559 (the entirety of which is incorporated herein by reference) discloses a phosphor film deposition method that utilizes two sputtering targets to deposit a rare earth activated alkaline earth thioaluminate phosphor film. One of the sputtering targets comprises aluminum and the other sputtering target comprises the remaining ingredients in the phosphor, typically one or more alkaline earth sulfides and a rare earth sulfide or oxide as a source of the activator species. The use of two sputtering targets facilitates modulation of the relative deposition rate of materials arising from each source which facilitates deposition of a laminated film with a periodic composition alternately rich and poor in aluminum. The variation is achieved by using a rotating or oscillating substrate that is alternately positioned in the flux of atomic species sputtered from the respective targets and the thickness of the layers can be altered by changing the rotation rate or the oscillation rate of the substrate. However, the composition modulation across the thickness of the deposited layer is problematical for subsequent reaction of the deposited materials to form a homogeneous single phase phosphor material, since atomic species are required to diffuse within the deposited film to achieve a homogeneous composition on an atomic scale.
U.S. Pat. No. 6,447,654 (the entirety of which is incorporated herein in its entirety) discloses the sputtering of thioaluminate phosphor films from a single target comprising aluminum sulfide and alkaline earth sulfides. The sputtering method can be used to deposit ternary and other chemically complex phosphor materials such as green-emitting magnesium calcium thioaluminate and blue-emitting barium magnesium thioaluminate phosphor materials by adjusting the target composition to account for differential condensation rates of the target elements on the phosphor film substrate. This method does not however, fully solve the problem of providing a phosphor film that can both be etched and is stable in display operation and at the same time be economically used for the deposition of phosphor films over large areas.
While there are several different methods to deposit phosphor materials, it is desirable to develop a method for the deposition of phosphor materials for thick film dielectric electroluminescent displays that obviates one or more of the disadvantages of the prior art methods. In particular, it is desirable to develop a novel deposition method whereby a homogeneous single phase phosphor material is deposited with minimal compositional variation over the surface and across the thickness of the deposited phosphor in a manner that is economical for large areas as is required for the fabrication of large electronic displays such as those for wall television applications.